Mucroserpula, Regenhardt, 1961

Mucroserpula View in CoL ? sp.

Fig. 10B View Fig .

Material.— Two specimens attached to an oyster shell and a belemnite from the middle Bathonian (Middle Jurassic) of Gnaszyn Dolny, Polish Jura (see Table 1); GIUS 8-3730 .

Description.—Tubes relatively small, up to 15 mm long, increase slowly in diameter (up to ca. 1 mm), straight and attached to the substrate along their entire length. Tubes with three slightly developed keels, of which the median one may be slightly undulating and with very delicate alae-type peristomes forming a short denticle above the aperture. The tube surface slightly rough and its base only gently widened. Cross-section pentagonal.

Remarks.—The specimens investigated are tentatively assigned to Mucroserpula due to their median and supralateral keels and resulting pentagonal cross-section. However, due to their slightly undulating median keel and relatively small size, the tubes may also belong to the genus Filogranula . Apart from the distinctive pentagonal cross-section and a short denticle marked above the aperture, both typical for the species of Mucroserpula , these tubes also resemble those of species of Propomatoceros from the same locality. The somewhat artificial classifi- cation of strictly single-keeled Propomatoceros and three-keeled Mucroserpula is not completely satisfactory, and it is highly subjective where to put a boundary between these genera. Cross-section and appearance of the keels may vary during ontogeny, making clear designations difficult. Additionally, diagenetic compression may also change the curvature and general outline. Although species of Mucroserpula are usually described as forming loop-like tubes (e.g., Ippolitov 2007b), coiling mode cannot be a primary indicative feature as the coiling may be facultative and strongly dependent on the kind of substrate and restriction of space for winding and coiling. Finally, the number of specimens described here is limited, hampering proper and unambiguous determination.